The relatively low deformation temperature of biaxially oriented PET excludes its
use in many hot fill applications or hot washing as part of a reuse cycle, due to the
shrinkage which generally occurs on reheating towards Tg.
Isothermal, isometric annealing processes (referred to as heat-setting) can cause
oriented PET to maintain its dimensional stability to higher temperatures.
However, to experiment on a commercial stretch blow moulding machine is a very
costly and time consuming process, justifying the requirement for an accurate
simulation technique. The objective of this project was, therefore, to simulate the
stretch blow moulding of PET with a heat-setting phase, by biaxially drawing PET
sheets on a TM Long Stretcher, then to heat-set them in a separate phase using a
purpose designed jig.
A heat-setting rig was designed and constructed at IPTME. Due to the size of the
proposed investigation window, a factorial experimental design (FED) technique
was employed to characterise the behaviour of two commercial bottle grades of
PET (B90N and B95A Laser) under a variety of draws and heat-setting conditions.
These sheets were then characterised in terms of shrinkage, tensile properties and
resistance to creep. This data was regressed to generate predictive model
equations for both grades of material and in both major and minor draw axes. Using these equations, it is now possible to define a set of processing parameters
which would give a bottle with lower and more uniform shrinkage (less draw
dependent) with enhanced creep properties and at least comparable mechanical
properties.
To develop a more fundamental understanding of the process of heat-setting in
biaxially drawn PET, as well as to prove the validity of the model equations, a
separate series of conventional experiments was devised to investigate the two
most significant predictor variables from the FED analysis; biaxial draw ratio and
heat-setting temperature (for Laser only).
This material was characterised by TMA, DSC and birefringence measurements.
The DSC and birefringence measurements allow the effect of heat-setting and draw
on crystallinity levels and orientation to be monitored. Orientation of both phases
was studied in heat-set bottles using micro polarised FTIR.

Description:

A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.